Hyaluronic acid is a member of a class of polymers known as glycosaminoglycans. It is a naturally occurring linear polysaccharide composed of alternating N-acetyl-D-glucosamine and D-glucuronic acid monosaccharide units linked via β-1,4-bonds, with the disaccharide units linked via β-1,3-glycoside bonds. Hyaluronic acid usually occurs as salts such as sodium and potassium hyaluronates. The sodium salt has a molecular formula of (C14H20NNaO11)n where n can vary according to the source, isolation procedure and method of determination. The molecular weight generally falls between about 6×104 and about 1.4×107. The term “hyaluronan” (HA) usually refers to both hyaluronic acid and its salts.
HA is present in the synovial fluid and the vitreous body of the eye. It also is widely distributed in the connective tissues of vertebrates, such as human umbilical cord, rooster combs and joint cartilage. HA can be isolated from all of these sources. It also can also be biosynthesized, and may be obtained using fermentation methods. For example, U.S. Pat. No. 5,411,874 describes a method for producing hyaluronic acid by continuous fermentation of Streptococcus equi. 
HA is non-immunogenic and non-toxic. When implanted or injected into a living body, however, HA typically is degraded by oxidation and by enzymes such as hyaluronidase. Because HA is a water-soluble polymer and is degraded and eliminated rapidly in vivo, the potential applications for HA in biomedical purposes have been somewhat limited.